The primary genetic risk factor for Alzheimer?s disease (AD), the apolipoprotein E ?4 allele (APOE4), disproportionally affects women. Studies in mice have shown that APOE4 promotes cognitive deficits and AD- like pathology that are significantly more pronounced in females. Despite this strong association between female sex and APOE4, there is a striking gap in our knowledge on how APOE4 imparts this female bias for AD risk. One mechanism underlying neural sex differences is developmental sexual differentiation, which results in permanent structural and functional alterations in the brain. Our central hypothesis is that sexual differentiation yields a female brain that is inherently more vulnerable to the AD-promoting actions of APOE4, a relationship that increases with age. How the female brain affects APOE4 function is unknown, though at least two critical components of AD pathology likely contribute to the increased AD risk in female APOE4 carriers. Specifically, we posit that interactions among APOE4, female sex, and AD pathology are modulated by: (i) microglia-induced neuroinflammation, and (ii) isoform-specific apoE lipidation and consequent changes in amyloid-? (A?) solubility. We propose 4 Specific Aims to investigate these hypotheses. In Aim 1, we will answer the critical questions of whether women are inherently more vulnerable to the effects of APOE4 and whether sexual differentiation contributes to this increased risk. We will investigate these issues using a large consortium of longitudinal human twins studies. In Aim 2, we propose rodent studies to complement the human twin studies. To address our central hypothesis, we will manipulate the sexual differentiation of EFAD mice (a transgenic mouse model of AD that contains both AD transgenes and the human APOE genotypes 3/3, 3/4, and 4/4) by creating masculinized females and feminized males to understand how cognitive impairment and the development of AD-like neuropathology are altered across the female ? male continuum in adulthood and aging. In Aims 3 and 4, we evaluate two mechanisms that are hypothesized to significantly contribute to the interactions among APOE4, sex, and AD pathology. In Aim 3, we investigate the hypothesis that sexual differentiation results in a female brain that is more vulnerable to the pro-inflammatory phenotype associated with APOE4, which promotes AD pathogenesis. In Aim 4, we investigate the hypothesis that APOE4 and female sex interact to reduce apoE lipidation, impairing clearance of soluble A?, resulting in memory/cognitive deficit and accelerated AD pathology. This proposal will provide novel insights into the critical link between female sex and APOE4-induced risk for AD and define the role of sexual differentiation in this relationship. Our complementary approach compares the female ? male continuum and the influences of 0,1, and 2 APOE4 alleles in both human twins and EFAD mice. In addition, we evaluate two potential mechanisms that may underlie these effects. Collectively, these studies promise to add significant depth to our understanding of the relationship between sex and APOE.